How does real life camera sensor size affect tracking on Blender?
If you use Automatic Refinement for the focal length during the solve process in Blender then it shouldn't really matter, since Blender will calculate the (more or less) correct focal length for the given sensor size. For the final image it doesn't really matter anyway, as long as aspect ratio and the camera solution itself are fine.  

But if you want to be sure you have the correct solution by comparing it with real world data, like your actual camera sensor size and focal length, you can fill in those values in the Camera and Lens panels of the clip editor and see if you still get a proper solution. If the automatic refinement calculated something that is way off from the real world data, then probably something is wrong.
Of course, as you described, you have to account for the crop factor. An iPhone will hardly have a true 35mm or so focal length, but rather something like 4.2mm. But if you take into account that it also is not a full frame camera, then it's really just about the correlation between focal length and sensor size that matters in the end. 

See this video, where I try to describe it:
https://www.youtube.com/watch?v=yjLTg37hFUM&amp;index=9&amp;t=0s&amp;list=PLtuvwW4VAp5tu2RdbRHThM6FVFfvFur1g